Multiple fluid injection nozzle array for rotary atomizer

ABSTRACT

A rotary atomizer for spraying fluids toward an object to be coated with the fluids includes an atomizer housing defining a central longitudinal axis and enclosing a plurality of feed tubes between an inlet end and an outlet end of the housing, and a shaft rotating about the longitudinal axis and protruding from an outlet end of the housing. A distributor is mounted on the shaft and spaced from the outlet end of the housing, for atomizing the fluids conveyed by the plurality of feed tubes and directing a spray of the fluids radially outward toward the object to be coated. A plurality of injection ports, each communicating with a corresponding feed tube is located at the outlet end of the housing for injecting the fluids toward the distributor. Each feed tube can be dedicated to a different fluid.

CROSS REFERENCES TO RELATED APPLICATION

The present application is a continuation-in-part of U.S. Pat. Nos.5,079,854, issued Jan. 14, 1992; 5,037,676, issued Aug. 6, 1991; and5,090,350, issued Feb. 25, 1992, corresponding respectively to U.S.patent application Ser. Nos. 457,958, 457,494 and 457,926, all filedDec. 27, 1989. These disclosures are herein incorporated by reference.

The present application also is technically related to U.S. patentapplication Ser. No. 07/684,382, filed on Apr. 12, 1991 by John M.Hammond, the disclosure of which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a rotary atomizer for coating substrates suchas drum and flexible belt photoreceptors for photocopiers. Moreparticularly, the invention relates to a rotary atomizer with a multiplearray of injection ports, each of which delivers a different fluid usedin the coating of the substrate.

2. Description of Related Art

A photoreceptor is a cylindrical or belt-like substrate used in axerographic apparatus. The photoreceptor substrate is coated with one ormore layers of a photoconductive material, i.e., a material whoseelectrical conductivity changes upon illumination. In xerographic use,an electrical potential is applied across the photoconductive layerwhich is then exposed to light from an image. The electrical potentialof the photoconductive layer decays at the portions irradiated by thelight from the image, leaving a distribution of electrostatic chargecorresponding to the dark areas of the projected image. Theelectrostatic latent image is made visible by development with asuitable powder. Better control of the coating quality yields betterimaging performance.

One method of coating substrates is to dip the substrate in a bath ofthe coating material. This method is disadvantageous because it usuallyresults in a non-uniform coating. In particular, when the substrate isoriented vertically and dipped into a bath, the coating thickness tendsto "thin" or decrease at the top of the substrate and "slump" orincrease at the base of the substrate due to gravity induced flow of thecoating material as the substrate is lifted from the bath. Thicknessvariations also occur even when the photoreceptor is orientedhorizontally and dipped into the bath due to the formation of a meniscusas the substrate is removed from the bath. This variation in coatingthickness causes variations in the performance of the photoreceptor. Inaddition, the dipping process requires additional processing controlsbecause the bath must be constantly maintained in a state suitable forcoating. The bath increases the size of the entire processing apparatusand is not readily adaptable to rapid changes in coating formulations.Further, changes in coating formulations are inhibited due toincompatibilities between formulations for successive coatings orlayers. It is also difficult to incorporate cleaning and curingoperations that are compatible with the dipping process for efficientmodular operation as a manufacturing process.

In another method, an air assisted automatic spray gun uses highvelocity air to atomize the coating formulation which is sprayed onto asubstrate. Due to high mass transfer rates intrinsic to the use ofatomizing air, this method entails considerable evaporative loss ofsolvent from the spray droplets and requires the use of slow evaporatingsolvents to prevent excessive solvent loss before the droplets arrive atthe substrate. It is difficult to use this method in a sealedenvironment, and thus difficult to control the solvent humiditysurrounding the substrates prior to, during, or after the coatingprocess. In addition, the air atomized spray method creates aconsiderable amount of overspray which results in higher material usage.Air spray guns also are less advantageous for batch processing of anumber of substrates.

Related Pat. Nos. 5,079,854; 5,037,676 and 5,090,350 disclose anapparatus for coating cylindrical and belt like substrates in which aplanetary array of substrates 18 (FIGS. 1A and 1B) is mounted on asupport structure 20 carried by a rotatable carousel 10. Each substrate-8 is rotated about a horizontal axis "h" while horizontally supportedabout a central horizontal axis H of the support structure 20. Thesupport structure 20 is inserted into a coating chamber 310 having areciprocating applicator 320 with its longitudinal axis aligned with thecentral horizontal axis H of the support structure 20 for applying acoating formulation radially outward into the planetary array ofsubstrates 18.

The applicator is a rotary atomizer having a housing 322 defining aninlet end 326 and outlet end 328. The housing 322 encloses a shaft 325rotating about the longitudinal axis and protruding from the outlet end328 of the housing 322. A distributor or "bell" 324 is mounted on theshaft 325 and spaced from the outlet end 328 of the housing.

As illustrated in FIGS. 2A, 2B and 2C, the housing 322 of commerciallyavailable rotary atomizers has a single feed tube 30 with one injectionnozzle 32 mounted at the end of the tube 30. The feed tube 30alternately delivers various coating solutions, or flushing solventsbetween coating runs. The fluid is delivered to the rotating bell 324where it is atomized (and electrically charged in cases where a highvoltage is applied to the bell) such that it directs a spray radiallyoutward. A batch delivery of a coating solution for a given layer isusually followed by a batch delivery of air and/or flushing solvent toclear the feed tube prior to delivery of a coating solution for thesubsequent layer. Efficient flushing of a coating solution from the lineby solvent and air is difficult. Limiting the amount of solvent and/orair used in flushing may result in coating solution residues remainingin the feed tube, which can contaminate subsequent coated layers,thereby degrading overall photoreceptor quality; while using largequantities of solvent and/or air for feed tube flushing can increasemanufacturing process cycle times to unacceptably high levels.

SUMMARY OF THE INVENTION

An object of the invention is to obviate the drawbacks of prior rotaryatomizers.

In accordance with the invention, a rotary atomizer for spraying fluidstoward an object to be coated, comprises:

an atomizer housing defining a longitudinal axis and enclosing a hollowshaft rotating about the longitudinal axis and protruding from an outletend of the housing and a plurality of feed tubes in the hollow shaftbetween an inlet end and the outlet end of the housing;

distribution means mounted on the shaft and spaced from the outlet endof the housing, for atomizing the fluids conveyed by the plurality offeed tubes and directing a spray of the fluids radially outward towardthe object to be coated; and

a plurality of injection ports located inside of said hollow shaft andaligned along said longitudinal axis, each communicating with acorresponding feed tube and being located at the outlet end of thehousing for injecting the fluids toward the distribution means.

Each injection port can then be dedicated to deliver an individualcoating solution or solvent or gas. The claimed invention provides atleast the following advantages:

1) No cross contamination occurs in the tubes. When the coating fluidsshare a common delivery tube to the atomizer, difficulties areencountered in the flushing of the tube between successive layercoatings. Precipitate sludges often form in the tube due tosolvent/polymer incompatibilities, resulting in deposition of sludgeparticles on photoreceptor surfaces, or total plugging of the commonfluid line;

2) No line flushing is necessary between layers. This eliminates therisk of contamination of the photoreceptor devices in process byflushing waste liquid;

3) Manufacturing process cycle time is reduced, resulting in greaterthroughput and lower unit manufacturing costs (UMC). The multi-tubearray enables all coating solutions and solvents to be primed and readyfor delivery before processing begins, thus eliminating various timeconsuming fluid delivery preparatory steps during processing;

4) Fabrication of unique device structures such as a single layer deviceor multipigment device are enabled. Sequenced deposition or blending ofcoating solutions or multicomponent reactive fluids becomesstraightforward; and

5) Flushing solvent waste volumes are minimized, since less lineflushing is required. Ecological benefits of this are obvious sinceexcess solvents are not released to the atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to thefollowing figures in which like elements are denoted with like referencenumerals and wherein:

FIG. 1A is a schematic cross-sectional top view of the coating chamber;

FIG. 1B is a schematic cross-sectional side view of the coating chambertaken along the lines X--X of FIG. 5A;

FIG. 2A is a schematic cross-sectional side view of a commerciallyavailable rotary atomizer into which the coating fluid is fed through asingle feed tube located along the atomizer central axis;

FIG. 2B is an enlarged view of the atomizer of FIG. 2A taken along theline A--A of FIG. 2A;

FIG. 2C is an end view of the tube of FIG. 2B taken along the line B--Bof FIG. 2B;

FIG. 3A is an enlarged view of the outlet end of the rotary atomizer inaccordance with the claimed invention;

FIG. 3B is an enlarged side view of the atomizer of FIG. 3A illustratingthe hollow shaft and inlet end;

FIGS. 4A, 4B, 4C and 4D are end views of different embodiments for thefeed tubes of FIG. 3 taken along the line C--C of FIG. 3;

FIGS. 5A, 5B, 5C and 5D are side views of different embodiments for thefeed tubes of FIG. 3 taken along the lines D--D of FIG. 3;

FIGS. 6A, 6B and 6C are a side view of an atomizer having anotherembodiment for a feed tube/nozzle assembly, an enlarged view of thenozzle assembly, and an end view of the nozzle assembly, respectively;and

FIGS. 7A, 7B and 7C are an end view of another embodiment of a feedtube/nozzle assembly, a cross-sectional side view of a portion of thefeed tubes, and a cross-sectional side view of the outlet end of thenozzle assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the preferred embodiment of the invention illustrated in FIGS. 3A and3B, the rotary atomizer 320 sprays fluids toward an object (preferably aphotoreceptor) to be coated with the solution. The atomizer has anatomizer housing 322 defining a central longitudinal axis and encloses aplurality of feed tubes 30A, 30B, 30C . . . terminating at the outletend 328 of the housing 322. A hollow shaft 325 is enclosed in thehousing and protrudes from the outlet end 328 by a spaced distance. Theshaft 325 rotates about the central longitudinal axis of the housing 322and encloses the feed tubes 30A, 30B, 30C as illustrated in FIG. 3B. Thehousing also accommodates a plurality of turbine blades (not shown).

A distributor or bell 324 is mounted on the shaft 325 and is spaced fromthe outlet end of the housing 322, for atomizing fluid delivered throughthe feed tubes 30A, 30B, 30C . . . and directing a spray of the fluidsradially outward.

A plurality of injection ports 32A, 32B, 32C . . . each communicatingwith a corresponding feed tube 30A, 30B, 30C . . . , is located at theoutlet end of the housing for injecting the fluids toward the bell 324.As illustrated in FIGS. 4A, 4B, 4C and 4D, the number of injection portsand corresponding feed tubes may vary from three (FIG. 4A), four (FIG.4B), seven (FIG. 4C), or more (FIG. 4D), up to about twenty tubes andports. Preferably, the injection ports are symmetrically arranged aboutthe central longitudinal axis.

In the preferred atomizer (a Nordson RA-12 available from NordsonCorporation of Amherst, Ohio but modified in accordance with thefollowing structure), a space of approximately 1 cm in diameter by 20 cmlong is available inside the turbine blades for the array of injectionports, i.e., the nozzle array. If the individual tubes were made smallenough, perhaps as many as twenty tubes could be fitted in this space.As few as two can be used or three as illustrated in FIG. 4A. It hasbeen found that seven tubes (FIG. 4C) is a convenient number due tocurrent process requirements and the symmetry of the array; i.e. withequal sized tubes, six may be symmetrically spaced in a hexagonalpattern around the remaining tube which is located in the center.

With regard to process requirements, the seven tubes enable multiplecoating solutions to be in the fluid tubes, primed and ready forcoating. Thus it is possible to have the process prepared to coat abatch of any one of several typical two or three layered organicphotoconductor drums: in the seven port array, two different undercoatfluids could be readied, as well as two different charge generatorfluids, and two different charge transport fluids, for example. (Othercombinations are possible.) The remaining port would likely be a solventinjector for bell flushing.

The two or more charge generator fluids could be chosen from visiblelight sensitive and infrared light sensitive materials, which aretypically used in light lens copiers and laser printers, respectively.Thus the entire system would have the flexibility to rapidly changebetween and alternately produce batches of different product sets. Thisflexibility is beneficial in that it enables just-in-time manufacturing,the advantages of which are widely understood.

In addition, this nozzle array enables the blending of two or morecharge generator materials or charge generator and charge transportmaterials to fabricate unique devices, which is the subject of therelated application Ser. No. 07/684,382, filed Apr. 12, 1991, thedisclosure of which is herein incorporated by reference.

Stainless steel hypodermic needle tubing can be used to fabricate thearray. This material is precision manufactured, solvent compatible, andreadily available. Individual tubes in the array can be tack weldedtogether for structural strength (for example, see welds W in FIG. 4C).

Teflon tubing can also be used. Its low electrical conductivity may bean additional advantage in the use of certain conductive materials, asthe rotary spray process is electrostatically assisted.

The tubes can be bundled tightly together and fixed in place by the useof solvent resistant sleeve or heat shrink tubing (for example, seesleeve or tubing S in FIG. 4A). They can also be potted in a solventresistant polymer. (Possibly an epoxy or polyurethane; for example seepolymer P in FIG. 4D.)

The nozzles 32A, 32B, 32C can also be fabricated in a short bundle, ortubular passageway made by boring through a solid plug of suitablematerial 40 (See FIG. 6A, 6B and 6C) which is preferably a solventresistant material. This short nozzle structure can then be equippedwith barbed or other suitable connections 42 for coupling to therequired length of fluid delivery tubing 30A, 30B, 30C, and fastened viathreads 41 or other suitable means to a fixturing tube 46 which locatesthe short nozzle structure as desired in close proximity to the bell(not shown in FIGS. 6A, 6B or 6C). FIGS. 6A and 6B illustrate threeinjection ports 32A, 32B, 32C, but more can be provided as illustratedin FIG. 6C.

In all cases it is desired to firmly fix the nozzle assembly to theturbine housing such that precise concentric and axial spatialrelationships are maintained between the nozzle assembly and therotating hollow shaft and between the nozzle assembly and the atomizerbell. This may be accomplished by incorporating into the nozzle assemblya portion of the outer surface which is dimensioned to provide aninterference fit with precise alignment of the assembly in the housing;or alternatively providing a flange at the inlet end of the nozzleassembly which can be affixed precisely to the housing with screws orother suitable means; or a combination of both fixing methods.

In another embodiment illustrated in FIGS. 7A, 7B and 7C, a solid roundrod 50 of preferably solvent resistent material is ground to formchannels or grooves 52 lengthwise on the peripheral surface of the rod.The grooves 52 may be formed on only a portion of the rod 50, but shouldextend to the outlet end of the rod. Additional aligned grooves may thenbe formed from the inlet end of the rod to complete the groovedpassageways 52 from the inlet end to the outlet end. The rod 50 may alsohave a central bore 54. A sleeve 56 is then fit over the rod to seal thegrooves 52 from each other and form the nozzle array at the outlet end.

It is further noted that the axial position of the nozzles relative tothe bell may affect performance of the atomizer. For example, themaximum distance between the nozzle array and the bell is 1/8 to 3/16 ofan inch and the minimum distance (depending on tolerances) is 1/1000 ofan inch. However, in a multiple tube array, interstices orlongitudinally oriented spaces between tubes may exist. Also, tubes inthe array that are not conveying fluid present an empty space. Theinterstices or empty tube spaces provide an area into which the sprayedsolution may migrate. For example, some highly volatile solutions mayvaporize and migrate into the empty tubes and interstices, thusdegrading atomizer performance. Experiments have shown that optimalatomizer performance is obtained by providing a space of 50 to 100thousandths of an inch between the nozzle array and the bell surface.Performance is further improved with a nozzle structure like that ofFIGS. 6A, 6B and 6C or FIGS. 7A, 7B, 7C which eliminates intersticesbetween adjacent tubes.

Special configurations of the ends of the tubes may be desirable tocenter their fluid injection points on the central axis of the atomizerbell which should coincide with the central axis of the housing. This istrue particularly with respect to those toward the outside or perimeterof the array. This will assure an even distribution of spray emanatingfrom the atomizer, which in turn results in equal coating thicknessesamong all devices in a batch. Means to direct the fluid exiting thetubes toward the exact center of the bell could include bevelling,bending, crimping and/or reducing the ends of the individual injectortubes, as illustrated in FIGS. 5A, 5B, 5C and 5D. In other words, theends of the injector tubes could be formed into individually configurednozzle structures.

For example, FIG. 5A illustrates a squared configuration in which thefluid spray (represented by arrows) is parallel to the centrallongitudinal axis. In FIG. 5B, the ends of the tubes of the perimetertubes 30A, 30B are longer than the central tube 30C and angled towardthe central longitudinal axis. This configuration can be made bybevelling the array or bending the perimeter tubes. FIG. 5C illustratesa configuration in which the diameter of the perimeter tubes 30A, 30B isreduced, and the smaller diameter tubes have a length greater than thecentral tube 30C. The smaller diameter tubes are bent toward the centrallongitudinal axis to direct the fluids toward the central longitudinalaxis. In FIG. 5D, the perimeter tubes 30A, 30B are bevelled like FIG.5B, but drilled so that the perimeter tubes direct their flows towardthe central horizontal axis. Although only three tubes 30A, 30B and 30Care shown in FIGS. 5A-5D, the configurations are applicable to arrayshaving more tubes.

With the present invention, each feed tube and injection port candeliver individual coating fluids to the photoreceptor, the differingcoating fluids being applied simultaneously through different tubes, orbeing applied sequentially for improved photoreceptor layering effects.In the former case, the fluids are mixed at the atomizer bell, thusimproving the mixing process and eliminating the need for mixture priorto entry into the atomizer. A coating fluid and solvent can be mixed inthis manner. In the later case, flushing of one tube with a solventprior to application of a coating fluid through a different tube isunnecessary since each tube is dedicated to a fluid source. Other tubesin the array can be dedicated to bell cleaning solvents or solvents forchamber humidification. The overall effect is a reduction in cycle timeand avoidance of flushing procedures and cross contamination of variouscoating fluids.

In another application, the multiple feed tube/multiple injection portatomizer in accordance with the invention is applicable to industriesthat apply different paint colors or overcoatings to different products,such as vehicle bodies in the automotive industry. The present inventionwill allow each feed tube to be dedicated to a different color, thusreducing the need to flush a feed tube and permitting rapid changes ofcolor.

The invention has been described with reference to its preferredembodiments which are intended to be illustrative and not limiting.Various changes may be made without departing from the spirit and scopeof the invention as defined in the following claims.

What is claimed is:
 1. A rotary atomizer for spraying fluids toward anobject to be coated with the fluids, comprising:an atomizer housingdefining a central longitudinal axis and enclosing a hollow shaftrotating about the longitudinal axis and protruding from an outlet endof the housing and a plurality of parallel non-coaxial feed tubes withinthe hollow shaft between an inlet end and the outlet end of the housing;distribution means mounted on the shaft and spaced from the outlet endof the housing, for atomizing the fluids conveyed by the plurality offeed tubes onto an outwardly facing inner surface of the distributionmeans and directing a spray of the fluids radially outward toward theobject to be coated; and a plurality of injection ports, eachcommunicating with a corresponding feed tube and being located at theoutlet end of the housing for injecting the fluids toward thedistribution means; wherein the injection ports are symmetricallyradially positioned about the central longitudinal axis of the housingto minimize a radial distance between the central longitudinal axis andthe injection ports.
 2. The atomizer of claim 1:wherein each feed tubehas an inlet communicating with a fluid source different from otherfluid sources for other inlets of other feed tubes.
 3. The atomizer ofclaim 1:wherein the number of feed tubes and corresponding injectionports is in the range of two to twenty.
 4. The atomizer of the claim1:wherein the number of feed tubes is in the range of three to seven. 5.The atomizer of claim 1:wherein an outlet of at least one of theinjection ports is angled to direct the fluid toward the centrallongitudinal axis.
 6. The atomizer of claim 1:wherein at least oneinjection port has an individually configured nozzle structure fordirecting fluid toward the central longitudinal axis.
 7. The atomizer ofclaim 1:wherein outlets of the injection ports in cross section aresymmetrically arranged in an array about the central longitudinal axis,the outlets on a perimeter of the array being configured to direct fluidtoward the central longitudinal axis.
 8. The atomizer of claim 1:whereinthe feed tubes are stainless steel hypodermic needle tubing.
 9. Theatomizer of claim 1:wherein the feed tubes are teflon tubing.
 10. Theatomizer of claim 1:further comprising securing means for securingtogether the feed tubes.
 11. The atomizer of claim 10:wherein thesecuring means are welds for one feed tube to an adjacent feed tube. 12.The atomizer of claim 10:wherein the feed tubes are bundled into anarray and the securing means is a sleeve surrounding a perimeter of thearray.
 13. The atomizer of claim 10:wherein the feed tubes are bundledinto an array and the securing means is a polymer encasing the array.14. The atomizer of claim 1, wherein outlets of the injection ports aredefined by bores extending through a solid material to eliminateinterstices between outlets.
 15. The atomizer of claim 1, wherein thefeed tubes are defined by a corresponding plurality of grooves in aperipheral surface of a rod and a sleeve fitted about the rod to sealone groove from an adjacent groove.
 16. The atomizer of claim 1, whereina distance between the nozzle ports and the distribution means isbetween 50-100 thousandths of an inch.
 17. The atomizer of claim 1,wherein the feed tubes are of equal cross-sectional area.